This document presents a case report of a rare occurrence of a large intra-articular ossicle in the knee joint of a 14-year-old male. Magnetic resonance imaging found a large ossicle impinging on the anterior cruciate ligament, causing lifting of the ligament anteriorly and laterally. Intra-articular ossicles in the knee are uncommon but can grow to a significant size, as seen in this case, producing mass effect and clinically mimicking an ACL injury. The etiology of intra-articular ossicles is unclear but may be congenital, traumatic, or degenerative in origin.